Packet data in a cellular network is often highly bursty in which occasional periods of transmission activity are followed by longer periods of no activity. A wireless device monitors the downlink transmissions in each subframe to receive uplink grants or downlink data transmission. Since the wireless device does not know in advance whether it is scheduled to read data in a particular subframe received by the wireless device, the wireless device is required to monitor base station transmissions for downlink control signaling. This is referred to as reception monitoring and is applicable to general cellular networks.
With respect to Long Term Evolution (LTE) and LTE-Advanced standards applied to a cellular network, reception monitoring is performed. In general, LTE standards use Orthogonal Frequency Division Multiplexing (OFDM) where the radio resources are divided into OFDM symbols in the time domain and in orthogonal narrowband sub-carriers in the frequency domain. The smallest radio-frequency element in LTE is called a resource element (RE) that consists of one OFDM symbol in time that spans 66.7 microseconds and one sub-carrier in frequency that spans 15 kHz. A resource element can carry one modulation symbol. The smallest unit that can be scheduled to a wireless device is defined as a Physical Resource Block (PRB) pair that consists of 12 subcarriers in frequency and two slots in time. Each slot consists of 6 to 7 OFDM symbols and a cycle prefix. The PRB pair spans one subframe in time that has a duration of one millisecond.
In LTE, the base station is referred to as an eNodeB (eNB) that schedules the downlink transmissions to wireless devices on a per-subframe basis. In addition to transmitting the wireless device traffic data, the eNB transmits downlink control information (DCI) to wireless devices that informs the wireless devices of the location of the PRB-pairs allocated to the respective wireless device in the Physical Downlink Shared Channel (PDSCH) and of the type of modulation and coding that the wireless device has to use to decode data received by the wireless device, in addition to other control information that is needed by the wireless device to decode the data. In LTE release 8, 9 and 10, DCI is conveyed only in Physical Downlink Control Channel (PDCCH). PDCCH is transmitted in the control region of the subframe which is located at the beginning of the subframe in up to the first four OFDM symbols. In LTE release 11, enhanced PDCCH (ePDCCH) is introduced where DCI may also be transmitted in the data region of the subframe that carries the data traffic for wireless devices.
The reception monitoring, i.e., monitoring of base station transmissions, is either performed continuously or discontinuously using discontinuous reception (DRX). In particular, the wireless device is required to perform reception monitoring in order to determine if there is downlink data intended for it. However, continuously monitoring the downlink channel results in high power consumption that reduces the wireless device's battery time. Referring to FIG. 1, to reduce wireless device power consumption, DRX cycle 10 is performed by the wireless device so that the wireless device cycles between awake states 12a-12n (collectively referred to as awake state 12) and sleep states 14a-14n (collectively referred to as sleep state 14) to preserve battery life. Further, there may be specific situations where the awake state can be extended longer than the ON duration shown in 12a, such as due to the detection of initial uplink or downlink transmission, due to the expectation of possible retransmission, and/or during contention resolution in random access. When the wireless device is in sleep state 14, the wireless device does not monitor, i.e., decode, any channel. In other words, the wireless device goes to “sleep” to save battery power.
However, when the wireless device is in awake state 12, the wireless device decodes all control channel candidates to determine whether there is data for the wireless device in the subframe. Control channel candidates refer to control channels that may or may not be assigned to the wireless device but that the wireless device nevertheless is required to decode as illustrated in FIG. 1. For example, the control channel candidates may be PDCCH candidates monitored by the wireless device, and are illustrated in Table 1.
Search space Sk(L)Number ofAggregationPDCCHTypelevel LSize [in CCEs]candidates M(L)Wireless Device-specific16621264828162Common41648162
With reference to FIG. 2, there is shown an existing single stage reception monitoring process for a wireless device implementing DRX cycle 10. The wireless device decodes all control channel candidates in a current subframes, i.e., decodes all twenty-two PDCCH candidates in the current subframe (Block S100). Such decoding involves several power consuming steps. These steps include performing convolutional decoding, verifying correctness of the decoding by checking the cyclic redundancy check (CRC), and then using the CRC and a Radio Network Temporary Identifier (RNTI), such as cell radio network temporary identifier (CRNTI), to determine if each PDCCH candidate is intended for the wireless device. The wireless device then determines whether it is configured to receive another subframe during the current awake state 12 (Block S102). If the wireless device determines it is configured to not receive another subframe during current awake state 12, i.e. wireless device 20 is reverting back to sleep state 14, the single stage reception monitoring process ends and the wireless device may enter sleep state 14 until the next awake state 12.
However, if wireless device determines it is configured to receive another subframe during the current awake state 12, the wireless device tags the other received subframe as the current subframe and repeats Blocks S100-S104 such that the wireless device continues to decode all control channel candidates of subframes received during the wireless device's current awake state. In other words, during each awake state 12, the wireless device is required to perform single stage reception monitoring by decoding all possible control channel candidates that maybe assigned to the wireless device even if there is no actual data intended for the wireless device. According to Table 1, this means that the wireless device has to decode twenty-two (22) PDCCH candidates (sum of the numbers in the last column in Table 1). For each PDCCH candidate, the wireless device is required to perform convolutional decoding, verify the correctness of the decoding by checking the cyclic redundancy code (CRC), and then use the CRC and an RNTI, such as CRNTI, in order to determine if the PDCCH candidate is intended for the wireless device.
Despite the power saving from DRX, the wireless device still wastes battery power as the wireless device is required to decode all possible control channel candidates that may be assigned to it in order to determine if the wireless device is scheduled to read data in a subframe. Therefore, in wireless communication networks such as LTE, the wireless device, in the awake state performs decoding of twenty-two PDCCH candidates in each subframe even if there is no data intended for the wireless device. This arrangement and process wastes the limited battery power of the wireless device, which serves as a critical limitation for wireless devices, especially low-powered wireless devices that are expected to be deployed in large numbers for the internet of things (IoT).